In many applications, it is desirable that the shape and/or symmetry of pulses within a high energy pulse train are stable from pulse-to-pulse. By way of example, but not limitation, one such application is the use of a high-energy, pulsed laser beam to melt an amorphous silicon film to induce crystallization of the film upon re-solidification, for the purpose of manufacturing thin film transistors (TFT's).
Laser crystallization of an amorphous silicon film that has been deposited on a substrate, e.g., glass, represents a promising technology for the production of material films having relatively high electron mobilities. Once crystallized, this material can then be used to manufacture (TFT's) and in one particular application, TFT's suitable for use in relatively large liquid crystal displays (LCD's). Other applications for crystallized silicon films may include Organic LED (OLED), System on a Panel (SOP), flexible electronics and photovoltaics. In more quantitative terms, high volume production systems may be commercially available in the near future capable of quickly crystallizing a film having a thickness of about 90 nm and a width of about 700 mm or longer.
Laser crystallization may be performed using pulsed laser light that is optically shaped to a line beam, e.g., laser light that is focused in a first axis, e.g., the short-axis, and expanded in a second axis, e.g., the long-axis. Typically, the first and second axes are mutually orthogonal and both axes are approximately orthogonal to a central ray traveling toward the film. An exemplary line beam for laser crystallization may have a beam width at the film of less than about 20 microns, e.g. 3-4 microns, and a beam length of about 700 mm, or larger. With this arrangement, the film can be scanned or stepped in a direction parallel to the beam width to sequentially melt and subsequently crystallize a film having a substantial length, e.g., 900 mm or more.
In some cases, e.g., sequential lateral solidification processes, it may be desirable to ensure that the silicon film is exposed using a beam having an intensity that is relatively uniform across the long-axis. For this purpose, homogenizers e.g. lenslet arrays (so-called fly's eye arrays) or diffusers are typically used in the projection optics downstream of the laser to produce a beam of uniform intensity. However, these homogenizers operate most effectively if the beam input to the homogenizer has a symmetrical intensity profile. Fluctuations in laser beam shape and symmetry may lead to a corresponding degradation in beam uniformity at the exit of beam homogenizers. This non-uniformity, in turn, can undesirably create regions of non-uniformly crystallized silicon.
Excimer gas discharge laser sources are capable of producing the high power pulses suitable for generating a laser crystallization line beam, as described above. For example, a typical excimer laser source may emit a beam having a cross-section with a short axis of about 3 mm and a long axis of about 12 mm. This beam can then be homogenized and shaped into the line beam, as described above. While the pulse shape and intensity symmetry along the short axis is typically stable and close to Gaussian, the intensity along the long axis is generally non-symmetrical and unstable from pulse to pulse. Thus, if untreated, these pulses may not be homogenized properly and may result in a line beam having undesirable intensity variations along its length.
With the above considerations in mind, Applicant discloses a beam mixer for increasing intensity symmetry along a selected axis of a beam and a laser source incorporating a beam mixer.